Conservation of tRNA and rRNA 5-methylcytosine in the kingdom Plantae

BACKGROUND: Post-transcriptional methylation of RNA cytosine residues to 5-methylcytosine (m⁵C) is an important modification that regulates RNA metabolism and occurs in both eukaryotes and prokaryotes. Yet, to date, no transcriptome-wide identification of m⁵C sites has been undertaken in plants. Plants provide a unique comparative system for investigating the origin and evolution of m⁵C as they contain three different genomes, the nucleus, mitochondria and chloroplast. Here we use bisulfite conversion of RNA combined with high-throughput IIlumina sequencing (RBS-seq) to identify single-nucleotide resolution of m⁵C sites in non-coding ribosomal RNAs and transfer RNAs of all three sub-cellular transcriptomes across six diverse species that included, the single-celled algae Nannochloropsis oculata, the macro algae Caulerpa taxifolia and multi-cellular higher plants Arabidopsis thaliana, Brassica rapa, Triticum durum and Ginkgo biloba. RESULTS: Using the plant model Arabidopsis thaliana, we identified a total of 39 highly methylated m⁵C sites in predicted structural positions of nuclear tRNAs and 7 m⁵C sites in rRNAs from nuclear, chloroplast and mitochondrial transcriptomes. Both the nucleotide position and percent methylation of tRNAs and rRNAs m⁵C sites were conserved across all species analysed, from single celled algae N. oculata to multicellular plants. Interestingly the mitochondrial and chloroplast encoded tRNAs were devoid of m⁵C in A. thaliana and this is generally conserved across Plantae. This suggests independent evolution of organelle methylation in animals and plants, as animal mitochondrial tRNAs have m⁵C sites. Here we characterize 5 members of the RNA 5-methylcytosine family in Arabidopsis and extend the functional characterization of TRDMT1 and NOP2A/OLI2. We demonstrate that nuclear tRNA methylation requires two evolutionarily conserved methyltransferases, TRDMT1 and TRM4B. trdmt1 trm4b double mutants are hypersensitive to the antibiotic hygromycin B, demonstrating the function of tRNA methylation in regulating translation. Additionally we demonstrate that nuclear large subunit 25S rRNA methylation requires the conserved RNA methyltransferase NSUN5. Our results also suggest functional redundancy of at least two of the NOP2 paralogs in Arabidopsis. CONCLUSIONS: Our data demonstrates widespread occurrence and conservation of non-coding RNA methylation in the kingdom Plantae, suggesting important and highly conserved roles of this post-transcriptional modification.